Probing Oxygen Transport in β-Cyclodextrin–Cyclohexane Crystal Nanochannels by Luminescence Quenching from Nanoseconds to Seconds

Abstract

Molecular transport in nanochannels can deviate from normal diffusion and exhibit different regimes depending on the observation timescale. However, resolving them from nanoseconds to seconds remains challenging. Here, oxygen transport in β -cyclodextrin–cyclohexane supramolecular crystals containing solvent-filled nanochannels is probed using room-temperature phosphorescence and fluorescence of aromatic luminophores. Their emission lifetimes, ranging from approximately 50 ns to 15 s, provide intrinsic time windows spanning more than eight orders of magnitude. Comparison of fluorescence and phosphorescence quenching for the same guests provides an internal control for probe identity. The effective bimolecular quenching rate constants decrease by approximately five orders of magnitude. Between approximately 50 ns and 10 ms, the dependence follows a power law, k q ∝ τ 0 -0.8 , followed by a nearly lifetime-independent plateau. The theory of diffusion-controlled reactions relates this dependence to the time-dependent oxygen flux and indicates subdiffusive transport at intermediate times with an effective anomalous exponent of approximately 0.4. A random-barrier model with quenched disorder provides a physical interpretation of this regime and its crossover to normal transport at long times. Thus, molecular excited states with different emission lifetimes enable luminescence quenching to resolve distinct temporal regimes of molecular transport under nanoscale confinement.